Janssen (J&J) vaccine a s non-replicating viral vector, it is a DNA genome vaccine that transcribes to mRNA inside the cell.

So basically a mRNA vaccine with an extra step, just like AstraZeneca:

"It is important to note, the Janssen COVID-19 Vaccine does NOT contain SARS-CoV-2 and cannot give an individual COVID-19.24 Additionally, the genetic material delivered by the viral vector does not integrate into a person’s DNA.9, 25 The DNA genome of the adenoviral vector is delivered to the nucleus and stays there epichromosomally (i.e. adenoviral vectors lack the machinery to integrate their genome into the host chromosomes, and hence are classified as non-integrating vectors). After the gene coding for the SARS-CoV-2 spike protein is transcribed, the mRNA goes to the cytoplasm where it is translated into the spike protein which is then further processed. All these steps are executed by the host cell machinery."

https://www.janssenmd.com/janssen-covid19-vaccine/pharmacology/mechanism-of-action/mechanism-of-action-of-the-janssen-covid19-vaccine